1. Field of the Invention
The present invention relates to an information reproducing method and an information recorder/reproducer.
2. Related Background Art
There have conventionally been proposed a variety of reproducing methods which reproduce information signals by detecting magnetized areas from magneto-optical media on which information signals are recorded at high densities by forming the magnetized areas. A reproducing method which was proposed by Koyata Takahashi et al. in Joint MORIS/ISOM '97, Tu-E-05 in particular is characterized in that it transfers a magnetized area formed on a memory layer to a transfer region formed on a displacement layer and detects the transferred magnetized area in a magnified condition. A reproducing method of this kind has thereafter been referred to as magnetic domain magnifying reproduction. It has been reported that the magnetic domain magnifying reproduction was capable of detecting a magnetized area formed on the memory layer even when it was smaller than a light spot of a reproducing light beam.
Description will be made below of the conventionally proposed magnetic domain magnifying reproduction. FIGS. 1A and 1B are partially enlarged diagrams of a magneto-optical medium 10 which is used for the magnetic domain magnifying reproduction. FIG. 1A is a top view, whereas FIG. 1B is a sectional view. The magneto-optical medium 10 consists of a substrate (not shown) and a magnetic layer 11 disposed on a surface of the substrate. The magnetic layer 11 is composed of three layers made of magnetic materials, that is, a memory layer 14 made of TbFeCo, a switching layer 13 made of GdFe and a displacement layer 12 made of GdFeCo. The memory layer 14 is a perpendicular magnetization film, whereas the switching layer 13 is an internal magnetization film at a temperature lower than T3 but a perpendicular magnetization film at a temperature higher than T3 and has a Curie temperature of T4. Furthermore, displacement layer 12 is an internal magnetization film at a temperature not exceeding T3 but a perpendicular magnetization film at a temperature higher than T3 and assumed to have a Curie temperature higher than T4. In the memory layer 14, circular magnetized areas R1, R2, R3, . . . having a diameter of 0.5 .mu.m which are magnetized downward as shown in FIG. 1B and enclosed by domain walls Q1, Q2, Q3, . . . , are formed in a row as well as surroundings thereof which are magnetized upward. These circular magnetized areas R1, R2, R3, . . . are formed by a recording method which displaces the magneto-optical medium 10 relative to a recording light beam while irradiating the magnetic layer 11 with a recording light beam which has an intensity modulated by information signals to be recorded and is condensed into a fine spot, and simultaneously applying a magnetic field to a location irradiated with the recording light beam in a definite direction (light modulation recording method).
Then, a principle of the magnetic domain magnifying reproduction will be described with reference to FIGS. 2A through 2D. Description will be made taking as an example a case where a magnetized area is detected from the magneto-optical medium 10 shown in FIGS. 1A and 1B by the magnetic domain magnifying reproduction. To detect the magnetized area, the magneto-optical medium 10 is first displaced relative to a reproducing light beam while irradiating the magnetic layer 11 of the magneto-optical medium 10 with the reproducing light beam. FIGS. 2A through 2D sequentially shows status changes which occur in the magnetic layer 11 as time elapses. An arrow A in the drawing indicates a displacement direction of the magneto-optical medium 10.
When the magnetic layer 11 is irradiated with the reproducing light beam as described above, it is partially heated, thereby forming an isothermal line indicating the temperature T3 and another isothermal line indicating the temperature T4 which are represented by numerals 15 and 16 respectively in FIGS. 2A through 2D. In a region outside the isothermal line 15 of the displacement layer 12 wherein temperature is lower than T3, the switching layer 13 and the displacement layer 12 are the internal magnetization films. In a transfer region 17 which is line 15 of the displacement layer 12 wherein temperature is higher than T3, the displacement layer 12 is the perpendicular magnetization film. Furthermore, switching layer 13 is the perpendicular magnetization film in a region between the isothermal line 15 and the isothermal line 16 where temperature is higher than T3 and lower than T4, but demagnetized in a region enclosed by the isothermal line 16 where the temperature is higher than T4. Both the displacement layer 12 and the switching layer 13 are subjected to exchange coupling with the memory layer 14 in the region between the isothermal line 15 and the isothermal line 16 where both the layers 12 and 13 are the perpendicular magnetization films, whereas the displacement layer 12 is not subjected to exchange coupling with the memory layer 14 in the region enclosed by the isothermal line 16 where the switching layer 13 is demagnetized.
In the status shown in FIG. 2A first, the magnetized areas R1, R2, R3, . . . which are formed on the memory layer 14 are not located right under a transfer region formed on the displacement layer 12 and the memory layer 14 located right under a transfer region 17 is magnetized upward. As a result of exchange coupling with the memory layer 14, the magnetization of the memory layer 14 is transferred to the transfer region 17, thereby magnetizing it upward. In addition, an area of the transfer region 17 which is enclosed by the isothermal line 16 is not subjected to exchange coupling with the memory layer 14, but follows the upward magnetization which is transferred and formed to and in the transfer region 17 due to exchange coupling of surroundings thereof since no cause for downward magnetization is constituted. When the magneto-optical medium 10 displaces with a time lapse, a portion of the magnetized area R2 formed on the memory layer 14 is partially located right under the transfer region 17, as shown in FIG. 2B. At this time, the portion of the magnetized area R2 which is located right under the transfer region 17 is transferred to the transfer region 17 due to exchange coupling, thereby forming a magnetized area Re2 which is magnetized downward and enclosed by a domain wall Qe2.
When the magneto-optical medium 10 displaces with a further time lapse, a portion of the magnetized area Re2 which is transferred and formed to and on the transfer region 17 enters the region enclosed by the isothermal line 16 from the front (left side in the drawing) of the isothermal line 16 as shown in FIG. 2C. At this stage, driving forces directed toward a higher temperature, i.e., toward a center of the transfer region 17, are exerted to portions of the domain wall Qe2 as indicated by arrows D. The domain wall Qe2 is restrained in the region between the isothermal line 15 and the isothermal line 16 where the displacement layer 12 is in exchange coupling with the memory layer 14, whereas the domain wall Qe2 is liable to be displaced by actions of the driving forces in the region enclosed by the isothermal line 16 where the displacement layer 12 is not in exchange coupling with the memory layer 14. When energy is imparted by applying a magnetic field having an adequate magnitude (for example, -110 [Oe]) in a direction corresponding to a magnetization direction of the magnetized area Re2 which is transferred and formed, the domain wall Qe2 can be prolonged and the magnetized area Re2 is magnified within the region enclosed by the isothermal line 16 as shown in FIG. 2D.
When the magneto-optical medium 10 displaces with a further time lapse and the magnetized area R2 formed on the memory layer 14 goes from the rear (right side in the drawing) of the isothermal line 16 completely to the outside of the isothermal line 16, the magnetized area Re2 magnified in the transfer region 17 is contracted and disappeared, thereby resuming a condition similar to that shown in FIG. 2A. The magnetized area which is transferred and formed to and on the transfer region 17 is magnified each time the magnetized areas R1, R2, R3, . . . formed on the memory layer 14 are displaced sequentially to the isothermal line 16 by repeating the operations shown in FIGS. 2A through 2D. The magnified magnetized area can be detected with a reflected light of the reproducing light beam by utilizing a magneto-optical effect. The magnetic domain magnifying reproduction described above makes it possible to detect the magnetized area formed on the memory layer 14 by transferring and magnifying the magnetized area to the transfer region of the displacement layer 12 even when the magnetized area is smaller than the light spot of the reproducing light beam.
For the conventional magnetic domain magnifying reproduction described above, the domain wall must be prolonged to magnify the magnetized area transferred and formed to and on the transfer region 17 and it is necessary for this purpose to impart a large energy to the domain wall. Furthermore, it is necessary to exert the driving forces in directions nearly perpendicular to the portions of the domain wall within the region enclosed by the isothermal line 16 to magnify the magnetized area, however the driving forces exerted to the domain wall are actually in directions which are in parallel with the domain wall more accurately at locations which are closer to the isothermal line 16 on the domain wall as indicated by arrows D in FIG. 2C. It is therefore impossible to displace the portions of the domain wall close to the isothermal line 16 along the isothermal line 16 only with driving forces obtained with a temperature gradient. For this reason, it is impossible to magnify the magnetized area in the transfer region 17 enclosed by the isothermal line 16 only by heating the magnetic layer 11 with heat generated by the irradiation with the reproducing light beam and a magnetic field having an adequate magnitude must be applied in the direction corresponding to the magnetization direction of the magnetized area.
However, such a method cannot magnify a magnetized area when a rear end (left side in FIGS. 2A through 2D) of the magnetized area passes the front end of the isothermal line 16 though it detects the magnetized area in a magnified condition when a front end (right side in FIGS. 2A through 2D) of a magnified area having a definite magnetization direction (for example, downward in the example described above) passes a front end (left side in FIGS. 2A through 2D) of the isothermal line 16. Accordingly, it was impossible for recording digital signals consisting of `0` and `1` to apply the method to a mark edge recording mode which permits further enhancing a recording density by alternately forming magnetized areas having different magnetization directions, corresponding front ends and rear ends of magnetized areas having different lengths to `1`, and corresponding other portions of the magnetized areas to `0` though the method is applicable to a mark position recording method which corresponds magnetized areas having a definite magnetization direction to `1` and corresponds areas between the magnetized areas to `0`. Even when the method is applied to the mark position recording mode, it is incapable of separately detecting a plurality of magnetized areas existing within the region enclosed by the isothermal line 16, thereby requiring forming the magnetized areas with sufficient intervals and being incapable of sufficiently enhancing a recording density.
Now, description will be made of a configuration of a conventional magneto-optical recorder/reproducer.
FIG. 3 is a diagram illustrating a conventional optical head which records/reproduces information on a magneto-optical medium (magneto-optical disk). In FIG. 3, a reference numeral 40 represents a semiconductor laser used as a light source. A diverging light bundle emitted from the semiconductor laser 40 is collimated by a collimator lens 41 and shaped by a beam shaping prism 42 into a parallel light bundle which has a circular sectional shape. It is assumed here that linearly polarized component which are perpendicular to each other as a P component and a S component, and that the parallel light bundle is a linearly polarized light bundle composed of the P component (in parallel with the paper surface). The light bundle composed of the P component is incident on a polarized light beam splitter 43. The polarized light beam splitter has characteristics, for example, of transmittance of 60% and reflectance of 40% for the P component, and transmittance of 0% and reflectance of 100% for the S component. The light bundle of the P component which has transmitted through the polarized light beam splitter 43 is condensed by an objective lens 44 to project a fine light spot to a magnetic layer of a magneto-optical disk 45. An external magnetic field is applied from a magnetic head 46 to a portion irradiated with the light spot to record a magnetic domain (mark) on the magnetic layer.
Reflected rays from the magneto-optical disk 45 are returned by way of the objective lens 44 to the polarized light beam splitter 43, which splits a portion of the reflected rays and leads it to a reproducing optical system. The reproducing optical system further splits the split light bundle with a polarized light beam splitter 47 which is prepared separately. The polarized light beam splitter 47 has characteristics, for example, of transmittance of 20% and a reflectance of 80% for the P component, and transmittance of 0% and reflectance of 100% for the S component. One of light bundles split by the polarized light beam splitter 47 is led by way of a condenser lens 53 to a half prism 54 and split into two light bundles, one of which is led by way of a knife edge 56 to a photodetector 57. Error signals for automatic tracking and automatic focusing light spots are generated by these control optical systems.
The other light bundle which is split by the polarized light beam splitter 47 is lead to a 1/2 wavelength filter 48 for turning a polarization direction of the light bundle 45 degrees, a condenser lens 49 for condensing the light bundle, a polarized light beam splitter 50, and photodetectors 51 and 52 which detects light bundles split by the polarized light beam splitter 50, thereby reproducing information. The polarized light beam splitter 50 has characteristics of transmittance of 100% and reflectance of 0% for the P component, and transmittance of 0% and reflectance of 100% for the S component. Signals detected with the photodetectors 51 and 52 are differentially detected with a differential amplifier (not shown) to generate reproduced signals.
Data is recorded on the conventional magneto-optical medium dependently on difference in perpendicular magnetization directions. When the magneto-optical medium on which the information is recorded dependently on difference in magnetization directions is irradiated with a linearly polarized light, a polarization direction of a reflected light is turned clockwise or counterclockwise dependently on the difference in magnetization directions. It is assumed, for example, that a linearly polarized rays incident on the magneto-optical medium are polarized in a direction of an axis P of a coordinates system as shown in FIG. 4, a reflected ray corresponding to downward magnetization is polarized in a direction R+ which is rotated +.theta.k and a reflected ray corresponding to upward magnetization is polarized in a direction R- which is rotated -.theta.k. When an analyzer is placed in a direction shown in FIG. 8, rays transmitting through the analyzer are A and B for R+ and R- respectively, whereby information can be obtained as a difference in light intensity by detecting the rays with photodetectors. In the example shown in FIG. 3, the polarized light beam splitter 50 functions as an analyzer at 45 degrees from the axis P for one of the split light bundles and at -45 degrees from the P axis for the other light split of bundles. In other words, signal components obtainable with the photodetectors 51 and 52 are in phase reverse to each other, whereby reproduced signals can be obtained with reduced noise by differential detection of individual signals.
On the other hand, there have been in the recent years enhanced demands for higher recording densities on magneto-optical media as described above. Line recording densities on optical disks such as magneto-optical media are generally dependent on laser wavelengths of reproducing optical systems and NAs (numerical apertures) of objective lenses. Speaking concretely, a limit of a reproducible magnetic domain lies on the order of .lambda./2NA since a diameter of a light spot is determined once a laser wavelength .lambda. of a reproducing optical system and an NA of an objective lens are determined. To record information at a high density on the conventional optical disk, it is therefore necessary to shorten a laser wavelength of a reproducing optical system or enlarge an NA of an objective lens. However, improvements in laser wavelengths and NAs of objective lenses are also limited, and there have been developed techniques to enhance recording densities by contriving compositions of recording media and reading methods.
For example, the inventor has proposed in Japanese Patent Application Laid-Open No. 6-290496 a domain wall displacement reproducing method which transfers a magnetic domain recorded as a vertically magnetized domain on a memory layer to a displacement layer by projecting a light spot to a magneto-optical medium consisting of a plurality of laminated magnetic layers and magnifies the domain larger than the magnetic domain of the memory layer by displacing a domain wall of the magnetic domain transferred to the displacement layer, thereby reproducing information. This domain wall displacement reproducing method will be described with reference to FIGS. 5 through 8A and 8B. FIG. 5 is a diagram illustrating a configuration of a magneto-optical information recorder/reproducer which uses the domain wall displacement reproducing method.
In FIG. 5, a reference numeral 58 represents a magneto-optical recorder/reproducer which is connected to an information processor such as a computer (not shown) and an information recorder/reproducer such as a video camera. Disposed in the recorder reproducer 58 is a control circuit 59 which controls the recorder/reproducer as a whole. The control circuit 59 controls information transception byway of the external information processor and an interface controller 69, controls information recording and reproduction on a magneto-optical disk 61 by controlling internal components, and controls other operating components. A reference numeral 60 designates a spindle motor which rotates the magneto-optical disk 61 and is controlled by a spindle motor controller 68.
The magneto-optical disk 61 is configured to be set and removed into and out of the magneto-optical information recorder/reproducer 58 with a mechanism (not shown). A reference numeral 62 represents an optical head which optically records and reproduces information on the optical disk 61 and a reference numeral 63 designates a magnetic head which is located on a side opposite to the optical head 62 with regard to the magneto-optical disk 61 and applies a recording magnetic field for recording information. A reference numeral 64 denotes an optical head-magnetic head control circuit which controls a location of a light spot projected from the optical head 62 and a location of the magnetic head 63. This control circuit 64 performs automatic tracking control, seek control and automatic focusing control. A reference numeral 65 denotes an information recording circuit which records information and a reference numeral 67 represents an information reproducing circuit which reproduces information.
FIGS. 6A through 6D are schematic diagrams descriptive of a domain wall displacement reproduction type magneto-optical medium (the magneto-optical disk 61) used in the magneto-optical information recorder/reproducer shown in FIG. 5 as well as functions of the magneto-optical medium. FIG. 6A is a schematic sectional view of the magneto-optical medium and FIG. 6B is a schematic front view of the magneto-optical medium. The magneto-optical medium is composed of three magnetic layers 71, 72 and 73 which are a first magnetic layer (memory layer), a second magnetic layer (switching layer) and a third magnetic layer (displacement layer). A reference numeral 74 represents a light spot for reproduction and a reference numeral 75 designates an information track on the magneto-optical medium. Arrows in the layers indicate directions of atomic spins and domain walls 76 are formed between regions in which the directions of the atomic spins are reverse to each other. Used as the magneto-optical medium is a medium disclosed by Japanese Patent Application Laid-Open No. 6-290496 mentioned above.
FIG. 6C is a graph showing a temperature distribution formed in the magneto-optical medium. This temperature distribution is induced on the medium by a light beam (the light spot 74) projected for reproduction. Additional heating means (a heating light spot or the like) may be used to raise temperature of an area located before the light spot of the reproducing light beam and locate a peak of the temperature distribution after the light spot. It is assumed that a temperature of the second medium is Ts which is in the vicinity of a Curie temperature of the magnetic layer 72 at a position Xs.
FIG. 6D is a graph showing a distribution of domain wall energy density .delta.1 of the third magnetic layer 73 corresponding to the temperature distribution shown in FIG. 6C. When the domain wall energy density .delta.1 has a gradient in an X direction as shown in FIG. 6D, a force F1 shown in FIG. 6D is exerted to the domain wall of each layer which exists at a position X, functioning to displace the domain wall to a side on which a domain wall energy is lower. Since the third domain wall 73 has a low domain wall coercivity and a high degree of displacement of domain wall, its domain wall can be displaced easily and independently by the force F1. In an area which is before (on the right side in the drawing) the position Xs where the medium is set at a temperature lower than Ts, however, the domain wall in the third domain wall 73 is fixed at a position corresponding to the domain wall in the first domain wall 71 due to exchange coupling with the first domain wall which has a high domain wall coercivity.
When a domain wall 77 is located at the position Xs of the medium at this stage as shown in FIG. 6D, the medium is heated to the temperature Ts which is in the vicinity of the Curie temperature of the second magnetic layer 72, thereby breaking the exchange couplings of the second domain wall 72 with the first magnetic layer 71 and the second magnetic layer 73. As a result, the domain wall 77 of the third magnetic layer 73 momentarily displaces into an area where temperature is high and a domain wall energy density is low as indicated by an arrow. When the reproducing light spot 74 passes, all atomic spins are set in a direction in a magnetic layer 73 of the third magnetic layer which is located within the light spot. As the medium displaces, a domain wall 76 momentarily displaces and all the atomic spins are reversed and set in a same direction. As a result, signals reproduced by the light spot always have a definite amplitude independently of a size of a magnetic domain recorded on the first magnetic layer 71, thereby solving a problem of waveform interference due to an optical limit of diffraction. This method is capable of reproducing magnetic domains having sizes which are smaller than a limit of resolution on the order of .lambda./2NA which is determined by a laser wavelength .lambda. and an NA of an objective lens.
FIG. 7 is a diagram exemplifying an optical head used in the recorder/reproducer shown in FIG. 5. Shown in FIG. 7 is a two-beam optical head which projects a reproducing light spot and a heating light spot. In FIG. 7, a reference numeral 79 represents a recording/reproducing semiconductor laser which has, for example, a wavelength of 780 nm. A reference numeral 80 designates a heating semiconductor laser which has, for example, a wavelength of 1.3 .mu.m. Both the lasers are disposed so as to be incident on the recording medium as P component. Since laser beams emitted from semiconductor lasers generally have elliptic sectional shapes, it is conventional to obtain circular light spots on recording media using beam shaping prisms and nearly circular apertures.
Laser beams emitted from the semiconductor lasers 79 and 80 are shaped so as to have nearly circular sectional shapes by beam shaping means (not shown) and made into parallel light bundles by collimator lenses 81 and 82 respectively. A reference numeral 83 represents a dichroic mirror which is configured to allow a light bundle of 780 nm at 100% and reflects a light bundle of 1.3 .lambda.m at 100% and a reference numeral 84 designates a polarized light beam splitter which transmits the P component at 70 to 80% and reflects S component which is perpendicular to the P component at approximately 100%. The parallel light bundles emerging from the collimator lenses 81 and 82 are incident on an objective lens 85 by way of the dichroic mirror 83 and the polarized light beam splitter 84.
The light bundle of 780 nm is configured to be larger than an aperture of the objective lens 85, whereas the light bundle of 1.3 .mu.m is configured to be smaller than the aperture of the objective lens 85. Accordingly, an NA of the objective lens 85 serves less for the light bundle of 1.3 .mu.m, whereby the light bundle of 1.3 .mu.m forms a light spot larger than that of the light bundle of 780 nm on the recording medium 61. A reflected light bundle from the magneto-optical medium 61 is made again into a parallel light bundle by the objective lens 85, reflected by the polarized light beam splitter 84 and obtained as a light bundle 87. After wavelength separation by an optical system (not shown), servo error signals and information reproducing signals are obtained from the light bundle 87.
FIGS. 8A and 8B are diagrams descriptive of operations to perform the domain wall displacement reproduction using the optical head shown in FIG. 7. FIG. 8A shows a reproducing light spot and a heating light spot on the magneto-optical medium. In FIG. 8A, a reference numeral 88 represents a recording/reproducing light spot having a wavelength of 780 nm and a reference numeral 89 designates a heating light spot having a wavelength of 1.3 .mu.m. A reference numeral 90 denotes domain walls of magnetic domains recorded at a land 91 and a reference numeral 92 represents a groove. Furthermore, a reference numeral 93 designates an area which is heated by the heating light spot 89. The recording/reproducing light spot 88 and the heating light spot 89 can be coupled with each other on the land 91 between the grooves 92 as shown in FIG. 8A. Accordingly, a temperature gradient can be formed on a displacing recording medium as shown in FIG. 8B. The temperature gradient and the recording/reproducing light spot 88 are in relationship which is shown in FIGS. 6A through 6D, thereby making it possible to displace domain walls.
The domain wall displacement reproduction adopts as a method to record information on a magneto-optical medium a magnetic field modulation method which permits enhancing a line density. Furthermore, the grooves 92 are demagnetized by annealing with a high temperature light spot to facilitate displacements of the domain walls. FIGS. 9A and 9B compare shapes of magnetic domains which are recorded by the magnetic field modulation method on a medium which is annealed with shapes of magnetic domains which are recorded by the magnetic field modulation method on a medium which is not annealed. FIG. 9A shows the magnetic domain recorded on the medium which is annealed, whereas FIG. 9B shows the magnetic domain recorded on the medium which is not annealed. In case of the magnetic domain shown in FIG. 9A, the grooves 92 are preliminarily annealed and demagnetized by the light spot at high temperature. A high temperature area 97 is formed when a light spot 94 is projected and magnetic domains 98 are formed in shapes like feathers of arrows when an external magnetic field modulated correspondingly to information to be recorded is applied from a magnetic head (not shown). FIG. 9B shows similar magnetic domains 102 which are formed in shapes like the feathers of arrows on the medium which is not annealed.
Comparing shapes of borders 99 and 100 between the magnetic domains 98 shown in FIG. 9A with shapes of borders 103 and 104 between the magnetic domains 102 shown in FIG. 9B, it will be understood that the borders 103 and 104 shown in FIG. 9B have shapes of feathers of arrows which have high curvature like a shape of the high temperature area 97, whereas the borders 99 and 100 have shapes which are nearly linear. It is considered that magnetic properties were destroyed stepwise in the vicinities of the grooves 92 by annealing, thereby making the domain walls to be displaced more easily and forming the linear shapes which are more stable.
Though description has been made above of the domain wall displacement reproduction method which uses the two-beam type optical head for easy understanding, it is actually desirable to reproduce information with a single-beam type optical head since the two-beam type optical head poses a problem of delicate adjustment and a problem of high operating cost. Description will be made of operations to reproduce the recording magnetic domains having the shapes of feathers of arrows shown in FIG. 9A with a single beam with reference to FIG. 10A through FIG. 11G. FIG. 10A is a sectional view of a magneto-optical medium 61 which is similar to that shown in FIG. 6A and FIG. 10B is a plan view as seen from a side from which a light spot is to be incident. The magneto-optical medium 61 is composed, like that shown in FIG. 6A, of a first magnetic layer 71, a second magnetic layer 72 and a third magnetic layer 73.
Furthermore, a reference numeral 95 represents a land of a track and a reference numeral 96 designates a groove. A reference numeral 105 denotes a reproducing light spot. A temperature distribution indicated by an oval isothermal line is produced on a recording medium by irradiating it with a light spot 105. It is assumed that the medium is to be displaced in a direction indicated by an arrow C. Arrows in the magnetic layers of the magneto-optical medium 61 indicate directions of atomic spins. An area which is represented by a reference numeral 108 in FIG. 10A is a high temperature area in which temperature is higher than a Curie temperature of the second magnetic layer (switching layer) 72 and the switching layer 72 is demagnetized. Accordingly, the first magnetic layer (memory layer) 71 and the displacement layer 72 are not in exchange coupling in the high temperature area 108 and the magnetic domains (marks) of the memory layer 71 are not transferred to the third magnetic layer (displacement layer) 73. In an area where exchange coupling force is active other than the high temperature area 108, the domains of the memory layer 71 are transferred to the displacement layer 73.
When the domain walls 106 and 107 of the magnetic domains recorded on the memory layer 71 are going to be located on a border between the low temperature area and the high temperature area 108, the domain wall 106 displaces toward the high temperature area in a direction indicated by an arrow D and the domain wall 107 displaces toward the high temperature area in a direction indicated by an arrow E. A reference numeral 109 represents an area in which the domain wall 106 displaces (slashed left side down) (herein after referred to as a pre-area) and a reference numeral 110 designates an area in which the domain wall 107 displaces (slashed right side down) (hereinafter referred to as a post-area). When the information is reproduced by the conventional differential detection, however, information of the domain wall 106 and that of the domain wall 107 are mixed with each other in the light spot 105, thereby making it impossible to reproduce wanted information.
This problems will be described in more detail with reference to FIGS. 11A through 11G. FIGS. 11A through 11F show a condition where a light spot 105 scans a land 95 on a track. A magneto-optical medium is displacing in a direction indicated by an arrow C as in FIG. 10A and a reference numeral 109 represents a pre-area and a reference numeral 110 designates a post-area. Let us further assume that an isolated magnetic domain 112 is recorded on the land 95 and that only the isolated land 95, for example, is magnetized upward and other magnetic domains are magnetized downward. Reference numerals 113 and 114 are domain walls formed on both sides of the isolated magnetic domain 112. FIG. 11G shows reproduced waveforms of differential signals obtained at these areas respectively.
First, FIG. 11A shows a case wherein the light spot 105 is located at a position apart from the isolated magnetic domain 112. In this condition, both the pre-area 109 and the post-area 110 are magnetized downward, and a differential detection signal is at a standard level at this time as shown in FIG. 11G. FIG. 11B shows a case where the light spot 105 comes near the isolated magnetic domain 112. In this condition, the domain wall 113 has not reached the pre-area 109 yet and the differential detection signal is at the standard level as in the case shown in FIG. 11A. FIG. 11C shows a case where the domain wall 113 has just entered the pre-area 109. In this condition, the domain wall 113 of the displacement layer 73 which is located in the pre-area 109 displaces toward the high temperature area and an area represented by a reference numeral 122 is magnetized upward. The differential signal is changed to a high level as shown in FIG. 11G.
FIG. 11D shows a case where the domain wall 114 on the opposite side has just entered the pre-area 109. The domain wall 114 of the displacement layer 73 which is located in the pre-area 109 displaces toward the high temperature area and returns to the condition where it is magnetized downward. The differential detection signal also returns to the standard level. FIG. 11E shows a case where the light spot 105 further advances and the domain wall 113 has just entered an end of the post-area 110. In this condition, the domain wall 113 of the displacement layer 73 which is located in the post-area 110 displaces toward the high temperature area and an area represented by a reference numeral 123 is magnetized upward. The differential detection signal changes to a middle level as shown in FIG. 11G. This signal level is lower than that corresponding to the pre-area 109 since a center of the high temperature area is located after a center of the light spot 105. FIG. 11F shows a case where the domain wall 114 on the opposite side has just entered the post-area 110. The domain wall 114 of the displacement layer 73 which is located in the post-area 110 displaces toward the high temperature area and returns to the condition where it is magnetized downward. The differential detection signal also returns to the standard level.
When the domain wall displacement reproduction method uses the single-beam type optical head as described above, a domain wall displaces in the two pre-area and post-area, thereby generating two pulses. In actual signals in which magnetic domains are optionally recorded, contributions by the displacements of the domain wall in the pre-area 110 and the post areas to the differential detection signal are mixed complicatedly with each other and cannot be separated as they are. To separate these contributions, there is available a method which suppresses displacement of the domain wall in the post-area by applying a magnetic field also at a reproduction time utilizing a difference between a degree of displacement of the pre-area and that of the post-area.
The domain wall displacement reproduction method which uses the single-beam type optical head requires, at a stage to manufacture a groove of media, preliminary annealing of the media, which constitutes a cause to make the media expensive. Though there is known the method which suppresses the displacement of the domain wall in the post-area by applying a magnetic field at a reproduction time, this method poses a problem to enhance power consumption. In the a case where media are not annealed, on the other hand, magnetic domains are recorded in shapes of feathers of arrows having high curvature on a memory layer as described with reference to FIG. 9B and when the magnetic domains are reproduced, the shapes of the feathers of arrows are not matched with those of borders at which a domain wall starts displacement, thereby making it impossible to displace the domain wall smooth. This problem will be described in detail with reference to FIG. 12.
Like FIG. 9B, FIG. 12 shows a condition where a recording magnetic domain 118 is recorded in a shape of a feather of an arrow on a land 91 by the magnetic field modulation method. A groove 92 is not annealed. When the land 91 is scanned by a reproducing spot 115, domain walls are displaced, thereby forming a pre-area 116 and a post-area 117. A reference numeral 121 represents a border which is to be used for staring displacement of the domain wall in the pre-area and has an arc-like shape having a center of curvature on the left side in FIG. 12. In contrast, borders (domain walls) 119 and 120 of a magnetic domain 118 have arc-like shapes having centers of curvature on the right side in FIG. 12. Since the arc-like shapes are curved in directions opposite to each other and remarkably different, the domain wall which is not annealed cannot displace smooth and displacement of the domain wall in the pre-area could not be reproduced. Though the domain walls 119 and 120 have shapes which are matched with that of the post-area, signal qualities are low and reproduced signals cannot be obtained since reproduced signals are originally low and the domain walls hardly displaces in the post-area as if a reproducing magnetic field were applied.